The number of television programs that a viewer may receive at any given time has increased geometrically in the last few years. Conventional analog television systems such as those conforming to the National Television Standards Committee (NTSC) and Phase Alternate Line (PAL) standards transmit one program per 6 MHz or 8 MHz channel. Recently, digital television signal processing techniques have been developed that allow multiple programs to be transmitted in each 6 MHz channel. Furthermore, there are now many sources of television signals In addition to the conventional terrestrially broadcast signals, a viewer may receive television signals, for example, via wired cable systems, several different types of satellite systems, so-called wireless cable systems and, in the near future, via a global information network, such as the Internet.
Conventional cable television systems are capable of delivering 140 six MHz channels and some systems are capable of delivering over 200 channels via a coaxial cable. Presently, new technology is being investigated to increase the number of programs that can be delivered to the home. This is being done via two technologically strategic moves. The first is to increase the allocated bandwidth to 1 GHz (which provides for 150 six MHz channels). The second is to use video compression to configure a channel to carry up to 10 minor channels in one 6 MHz wide major channel. Channels that include a plurality of minor channels are also known as multiprogram channels. Typical numbers that are used in the industry estimate that about 500 programs can be delivered to the home over a single coaxial cable. Fiber optic cable provides many times the bandwidth of a coaxial cable and promise to be able to provide several thousand programs. In the same way, increased bandwidth for satellite systems may allow a viewer to receive upwards of 1,000 six MHz channels, each channel containing up to ten minor channels.
The digital coding that allows a single television channel to contain more than one minor channel also makes the channels more difficult to decode. While an analog television receiver may be able to tune to a new channel and recover a television signal in a matter of milliseconds, a DTV receiver needs more time. This is because a typical digital television signal is first encoded according to the standard adopted by the Moving Picture Experts Group (MPEG) to provide a digital bit-stream. The digital bit stream is then used to modulate a radio frequency (RF) carrier using quadrature amplitude modulation (QAM) or trellis coded vestigial side-band (VSB) modulation techniques. In order to receive a program transmitted on a DTV channel, therefore, a receiver must tune the receiver to the channel frequency, phase-lock the tuner to the DTV signal, demodulate the signal and then decode the demodulated signal. For a typical DTV signal, this process may take four to ten seconds for each major channel.
Television receivers manufactured in the last few years typically include a tuning memory that stores tuning information for active channels which may be received by the television receiver. This information is commonly referred to as a channel map. The tuning memory may store, for example, a list of channels for which valid RF television signals are present and tuning information that may be used to set the television tuner to quickly recover the RF carrier. This list is used to step sequentially through channels when a viewer is watching television. As a viewer presses the channel up key on the remote control, a microprocessor in the television receiver accesses the channel map and provides information for the next channel in the list to the tuner.
In a typical analog television receiver, the channel map is derived using a set-up function accessed via the control menu of the receiver. The set-up function sequentially scans the tuner from the lowest channel frequency to the highest channel frequency, attempting to tune each channel as it is encountered. Only channels that provide valid television signals are added to the channel map.
While this system works well for analog television receivers, it may not be appropriate for DTV receivers. First, as described above, it takes considerably more time for a digital television decoder to determine if a valid signal exists at a channel frequency. Second, the existence of valid programming at the various channel frequencies may change more often in a DTV environment, such as a satellite broadcast, than in a traditional terrestrial broadcast environment, for example, due to changing weather conditions or sun-spot events. These events may affect the strength of the received satellite signal and make some of the channels temporarily unavailable. Thus, for a DTV receiver if conventional methods are used to generate a channel map, not only will it take more time but it will need to be done more often.